Intrinsic properties of rat striatal output neurones and time-dependent facilitation of cortical inputs in vivo

1. In vivo intracellular recordings were performed from striatal output neu rones (SONs) (n = 34) to test the role of their intrinsic membrane properti es in the temporal integration of excitatory cortical synaptic inputs. 2. In a first series of experiments, intracellular injection of a test depo larising current pulse was preceded by a 200 ms suprathreshold prepulse, ti tle two pulses having the same intensity. An increase in intrinsic excitabi lity was observed as a decrease (55 +/- 21 ms, n = 13) (mean +/- S.D.) in l atency to the first action potential of the test response compared to the p repulse response. This value decayed exponentially as a function of the tim e interval between the current pulses (tau = 364 +/- 37 ms, n = 5). The vol tage response of SONs was not modified by a prepulse that induced a membran e depolarisation < -62 mV. 3. The effect of the suprathreshold prepulse was tested on monosynaptic cor tically evoked excitatory postsynaptic potentials (EPSPs). The ability to i nduce suprathreshold EPSPs was markedly increased by the prior depolarisati on (n = 11 cells). This facilitation decayed progressively as a function of the time intervals between prepulses and cortical stimuli. The potentiatio n was not observed on small EPSPs reaching a peak potential < -65 mV (n = 3 ). 4. We conclude that SONs can optimise cortical information transfer by modi fying their intrinsic excitability as a function of their past activation. It is likely that this time-dependent facilitation results, at least in par t, from the kinetics of a striatal slowly inactivating potassium current av ailable around -60 mV that recovers slowly from inactivation.